Maintaining stiffness or rigidity of bonded particulate tile articles under high humidity conditions has long been a problem in the particulate tile industry. This problem is of particular concern since the articles are typically used in ceilings and supported only around their perimeters. Humidity weakens the tile and due to the perimeter-only support often induces an unacceptable sag of the tile.
The resistance of particulate tiles to moisture has previously been improved by applying a coating comprised of a melamine-formaldehyde resin. However, use of this resin, due to its formaldehyde content, is of concern due to the existence of regulations involving worker exposure during production of the resin and its application to the particulate tiles. Moreover, the environmental hazards associated with the use of this resin does not end with the manufacturing process since tiles bearing this coating release noxious and undesirable vapors under conditions such as those encountered in fires. Less noxious materials which exhibit satisfactory performance with regard to preventing humidity-induced sag of the particulate tiles to which they are applied have not heretofore been found.
U.S. Pat. No. 4,611,445 discloses a ceiling tile resistant to sag hen exposed to a fire comprising mineral wool fibers having distributed therein from about 1% to about 6% by weight, based on the weight of the fibers, of lithium carbonate or the fused decomposition product thereof. This coating, however, is not useful in preventing humidity-induced sag.
Various methods have been utilized in the corrugated paper art to reduce the degradation of paper strength due to moisture contact. Impregnating paper with certain resins, e.g. ureas, melamines, and phenolics, is known, but the use of such resins has only limited utility in the manufacturing of particulate tiles because these resins cure at normal paper machine temperatures and tiles coated therewith become excessively brittle during their processing. Moreover, many of these processes use coatings as noxious as those of which we seek to avoid to use.
U.S. Pat. No. 4,400,480 relates to a method of preparing water resistant paperboard through use of a crosslinking additive for starch-based adhesive compositions. These compositions are prepared by reacting acetone and formaldehyde under aqueous alkaline conditions at 20.degree.-380.degree. C. in a molar ratio of 1 mole acetone: 2-5.5 moles of formaldehyde. An effective amount of free formaldehyde is reduced to about 0.1-2% by weight. The resulting reaction product can then be added to a starch-based adhesive or first mixed with dimethylol dihydroxy ethylene urea. Due to its formaldehyde content, this resin has the same environmental and exposure concerns discussed earlier and therefore does not represent an improvement over the currently practiced art.
U.S. Pat. No. 4,600,439 discloses a coating composition for paper or cardboard, comprising a fluidizing agent and an undepolymerized starch, optionally modified, a synthetic product, such as polyvinylalcohol, or a protein, such as casein or soya proteins.
U.S. Pat. No. 3,858,273 discloses a method of improving the wet rigidity of corrugated paperboard through its impregnation with a slow-curing glyoxal-based resin in the presence of a catalyst and, optionally, an extender, such as a starch.
U.S. Pat. No. 3,288,631 discloses a method for the production of non-woven fiber webs that are stable in the presence of water comprising a water-soluble bonding agent having both hydroxyl- and carboxyl-groups and a water-soluble resin and metal salt capable of reacting with the aforementioned groups.
The above-mentioned paper coating methods are inadequate for use in the production of sag resistant tiles due to their failure to impart adequate moisture resistance and/or rigidity to a tile, thereby allowing the tile to exhibit an unacceptable degree of humidity-induced sag.
Commonly assigned U.S. patent application Ser. No. 07/433,984 is directed to a process for the prevention of humidity-induced sag in bonded particulate articles which discloses the application to at least one side of said article a coting composition comprising water, a filler material and a binder resin which is the reaction product of glyoxal resin and a starch. The coating composition is then cured on said article. While the particle of this process does adequately prevent humidity-induced sag in the particulate articles to which it is applied, the inclusion of starch, a polymeric substance, in the coating composition causes minor processing difficulties due to the relatively high viscosity which it imparts.
It is therefore the object of the present invention to provide a process for the production of a particulate article having adequate protection against humidity-induced sag without encounting processing difficulties due to high viscosity coating compositions.